The development and acceptance of wind energy as a clean and productive source of alternative energy is proliferating. Wind energy can be captured by a wind turbine generator, which is a rotating machine that converts the kinetic energy of the wind into mechanical energy, and the mechanical energy subsequently into electrical power. Common horizontal-axis wind turbines include a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle by means of a shaft. The shaft couples the rotor either directly or indirectly with a rotor assembly of a generator housed inside the nacelle. A plurality of wind turbine generators may be arranged together to form a wind park or wind power plant.
Wind power plants are coupled to an electrical network or a power grid. In some situations, wind power plants are situated in rural areas, and infrastructure is sometimes inadequately set up to support the wind power plant electrical distribution. As a result, such transmission grids which are far from the main generation units experience a phenomenon known as “weak grid”. The fault level at the Point of Connection (PoC) for a weak grid connected plant is low and Short Circuit Ratio (SCR) at the PoC is defined as SCR=(Fault level in MVA at the PoC)/(MW rating of the Wind power plant). In a weak grid, the SCR is generally less than 3.
A weak transmission grid or electrical grid or power grid or network grid is characterized by high grid impedance due to the low fault level. In a weak grid, it is typically necessary to take voltage level and fluctuations into account because there is a probability that the values might exceed the requirements in the standards when load and production cases are considered. Where a wind energy plant or a wind power plant is connected or coupled to a weak grid, due to the higher impedance of the grid, the amount of wind energy that can be pushed into the grid at the point of connection is limited because of a relatively large fluctuation of the voltage. Further, during normal operation, voltages in a weak grid can exceed the upper continuous operating voltage of the grid.
Transmission grids are typically governed by grid codes which regulate all power generating devices coupled to the grid for normalized operation. Wind power plants which are coupled on to a power grid fall under such regulation and are required to perform accordingly to a number of scenarios, one of which is that of staying connected to the grid during a fault event, for example, a low voltage fault event, and to contribute to the recovery of the power grid.
In a low voltage fault or a voltage dip event, the wind power plant is required to provide a reactive current contribution to support the grid voltage, as according to various grid codes. In certain grid codes, a generating plant connected to the transmission grid must maintain reactive current voltage support even after the grid voltage has returned to the normal continuous operating voltage band. For most grid code regulations, the normal continuous operating voltage band is from 0.9 pu to 1.1 pu.
In voltage dip occurrences in weak grids, and in an instance where a wind power plant is connected to the transmission grid, the wind power plant is expected to similarly provide a reactive current support in assistance to grid voltage recovery. Typically, a predetermined reactive current injection curve is provided for the provision of reactive current by a wind turbine generator or a wind power plant during the voltage dip. In situations where a recovery of a grid voltage is successful, the grid voltage of the transmission grid returns back to a reference voltage band of normal operation. However, according to various grid codes, reactive current support to the transmission grid is to continue even after the grid voltage recovers. It is in such a recovery situation, where for a weak grid, additional reactive current support can cause the wind turbine generator to trip due to exceeding a high voltage limit and if reached to the full active power generation enter a re-triggering loop due to voltage drop at the grid side.
There is as such a desire for a method for controlling a wind power plant operating in a weak grid environment which can continue to be connected to and supporting the grid through a grid fault occurrence.